refactor: move selection surgery into rich-text-dom.ts (RD-21)

deleteAdjacentChip and insert did getSelection()/Range work inside the
component, which pushed it over the max-lines budget under a disable
comment. rich-text-dom.ts already owns the DOM boundary, so the surgery
moves there as two new exports, chipAtCaret and insertChipAtCaret, and
the component keeps only its event-handling and output concerns.

adjacentChip stays exported with its own spec case. The component
disable comment is gone, since the file is now under the line budget.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 538 frontend behaviours across
+**is** the suite, reshaped for a business reader. 543 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -758,6 +758,14 @@ classes.
 - redirects an anonymous user to /login without waiting for caps
 - reads authentication through the port, not an app-local store
 
+#### chipAtCaret / insertChipAtCaret (selection surgery)
+
+- chipAtCaret returns the chip before a Backspace caret
+- chipAtCaret returns null for a non-collapsed selection
+- chipAtCaret returns null for a caret outside root
+- insertChipAtCaret splices at the caret and leaves the caret after the chip
+- insertChipAtCaret appends when there is no selection inside root
+
 #### createDebouncedSave
 
 - flushes after the delay when canSave is true